Pulse oximetry, a widely accepted noninvasive procedure for measuring the oxygen saturation level of arterial, blood, is responsive to pulsatile blood flowing within a fleshy tissue site. FIG. 1 illustrates the standard plethysmograph waveform 100, which can be derived from a pulse oximeter and corresponding pulse oximetry sensor. The sensor attaches to and illuminates a peripheral tissue site, such as a finger tip. The plethysmograph waveform 100 illustrates light absorption at the tissue site, shown along the y-axis 101, versus time, shown along the x-axis 102. The total absorption includes static absorption 110 and variable absorption 120 components. Static absorption 110 is due to tissue, venous blood and a base volume of arterial blood. Variable absorption 120 is due to the pulse-added volume of arterial blood. That is, the plethysmograph waveform 100 is a visualization of the tissue site arterial blood volume change over time, and is a function of heart stroke volume, pressure gradient, arterial elasticity and peripheral resistance. The ideal waveform pulse 130 displays a broad peripheral flow curve, with a short, steep inflow phase 132 followed by a 3 to 4 times longer outflow phase 134. The inflow phase 130 is the result of tissue distention by the rapid blood volume inflow during ventricular systole. During the outflow phase 130, blood flow continues into the vascular bed during diastole. The plethysmograph baseline 140 indicates the minimum basal tissue perfusion.
As shown in FIG. 1, a pulse oximetry sensor does not directly detect absorption, and hence does not directly measure the standard plethysmograph waveform 100. Rather, a pulse oximeter sensor generates a detected light intensity signal. However, the standard plethysmograph 100 can be derived from the detected intensity signal because detected intensity is merely an out of phase version of light absorption. That is, the peak detected intensity occurs at minimum absorption 136, and minimum detected intensity occurs at maximum absorption 138. Further, a rapid rise in absorption 132 during the inflow phase of the plethysmograph is reflected in a rapid decline in intensity, and the gradual decline 134 in absorption during the outflow phase of the plethysmograph is reflected in a gradual increase in detected intensity. A pulse oximetry sensor is described in U.S. Pat. No. 6,088,607 entitled Low Noise Optical Probe. A pulse oximetry monitor is described in U.S. Pat. No. 6,650,917 entitled Signal Processing Apparatus. Both of these patents are assigned to Masimo Corporation, Irvine, Calif. and incorporated by reference herein.